Various procedures are known for reducing the TDS content of an impure salt-laden water sufficiently to permit the water to be used for drinking or for industrial use, e.g. as a cooling water. Such processes include reverse osmosis, ion exchange, and electrodialysis. Where impure water containing highly concentrated amounts of salts or impurities are involved, other processes such as multiple-effect evaporation are employed.
Reverse osmosis involves the use of a semi-permeable membrane, such as the known cellulose acetate membrane, which, under the application of pressure to a feed stream, permits passage of water molecules but holds back or rejects all or part of the undesirable solute in the feed stream. Reverse osmosis is particularly applicable for removing or rejecting chloride ion from an aqueous feed solution containing such ion.
Certain ionic constituents or impurities in aqueous solutions can be removed by ion exchange, employing various types of ion exchange resins, as known in the art.
For removal of various types of ionic impurities in water, processes have been developed employing the combination of ion exchange resins and reverse osmosis. Thus, U.S. Pat. No. 3,639,231 discloses a process for desalination of brackish water by first treatment with ion exchange resins and then by reverse osmosis. The waste stream from the reverse osmosis treatment can be used to regenerate one of the ion exchange beds.
U.S. Pat. No. 2,771,418 pertains to a method of regenerating an ion exchanger used in the conversion of a nitrate salt solution and a chloride salt solution.
U.S. Pat. No. 3,836,458 relates to a water purification system wherein the water passes through a series of filters, the water then passes through a reverse osmosis unit, followed by passage through a mixed resin bed. This process produces deionized water which is not suitable for use in a public water supply system.
U.S. Pat. No. 3,823,068 discloses a pretreatment process for increasing the useful life of a reverse osmosis membrane, by passing a reverse osmosis influent initially through a strong basic anion exchange resin until carbonate ions are detected in the resin bed effluent, and passing such effluent through a reverse osmosis membrane.
However, the prior art as exemplified above has failed to develop, to applicant's knowledge, an efficient method for removing or substantially reducing the concentration of certain ionic impurities, especially nitrate ion, and nitrate type ions, as well as chloride ion, and reducing excessive TDS in impure aqueous solutions containing such ions or the corresponding salts, to produce quality drinking water suitable for distribution in public water supply systems.
It is accordingly one object of the present invention to purify water contaminated with impurities of the class of nitrates, arsenates, selenates, and borates. Another object is to provide a relatively simple and inexpensive process of the above type, wherein the above noted class of impurities are present in combination with chlorides. A still further object is the provision of efficient procedure employing a certain combination of reverse osmosis and anion exchange, for removing or substantially reducing impurities such as nitrates, arsenates, selenates and borates, particularly nitrates, in combination with chlorides, in aqueous solutions containing such impurities, and reducing excessive TDS, to provide a product water suitable for drinking, while producing a concentrated waste stream containing such impurities.